Magnetic disk devices, such as hard disk drives supporting random access, utilize a spindle that includes a collection of platters. These platters are covered with a magnetic material for recording information. Each platter contains a series of circular recording tracks containing sectors of information that can be read or written to by electromagnetic heads utilizing switchable magnetic fields. The platters of a spindle generally rotate at a constant angular speed when memory sectors are being read or written.
The rotation of a spindle of platters in a hard disk drive is effected by a spindle motor. The motor includes a magnetic rotor rotating in response to an electrical field created by three sets of electric coils. At any given point in time, only two of the three sets of coils are driven. As the rotor magnet sweeps past these coils, it generates on the undriven set of coils a back electromotive force (BEMF) signal which varies in intensity. The strength of the BEMF signal can be utilized by a hard disk drive controller to provide feedback about the speed and state of the spindle motor during operation.
In particular, the BEMF signal can be compared to a voltage at a center tap of the motor, which is a common potential tied to each of the three connected sets of motor coils, in order to adequately gauge the exact moment of commutation. This moment of commutation refers to the exact time when a given electric field produced by a set of coils turns on or off.
A problem arises when attempting to ensure that the spindle motor of a hard disk drive rotates at a constant speed specified by a hard disk drive controller. Previous techniques for ensuring that a spindle motor rotates at a designated constant speed include well known control techniques such as a phase locked loop control system to access and track the BEMF strength or phase. However, the phase locked loop detection technique cannot be easily resynchronized if it falls outside of a certain range of disparity between the intended and actual phase of the BEMF signal. Once the phase falls outside of the set range, the spindle motor must be powered off and/or started again from zero speed in order to resynchronize and accurately track a phase of the BEMF signal provided by the spindle motor. Additionally, the time required to alter the speed of rotation for the motor is high, due to a large capacitance in a low pass filter utilized by the phase locked loop technique. The capacitance in the low pass filter must be maintained at a large magnitude in order to reduce noise that routinely disrupts the circuit's performance.
The rotational speed of a spindle motor in a hard disk drive may change due to physical bumping, jarring, vibrations, the interruption of a power supply, an unclear supply line, or a power supply spike, for example. Thus, it may be seen that a method of synchronizing the speed of a spindle motor with a controller is needed that allows adjustment after a significant disruption, and that enables rapid adjustment of the rotational speed by a hard disk drive controller to ensure that the operation of the spindle motor of a drive meets desired performance criteria.
Another significant source of interference that has the potential to cause disruptions in the constant rotation of a spindle motor is the generation of noise by the spindle motor itself. Such noise can interfere with the ability of BEMF signal strength to accurately reflect the proper timing of applying inputs to a spindle drive motor. In particular, noise is created during the linear operation of a motor just after points of commutation corresponding to time intervals when a phase input to the spindle motor is altered. For a spindle motor operating in pulse width modulation mode, an additional source of noise and rotational disruption is found during time intervals when high frequency switching of a phase input occurs.